1. Field of the Invention
The present invention relates to a method of fabricating organic light emitting diode array, particularly to the one that adopts a directional spin coating technology.
2. Description of the Related Art
The characteristic of an organic light emitting diode (OLED) is that the material of its electro-luminescent (EL) layer is a small discrete organic molecule such as aluminum tris(8-hydroxyquinoline) (Alq3) or an organic polymer such as polyfluorene (PF). Refer to FIG. 1 the schematic diagram of a conventional OLED 1. As shown in FIG. 1, the conventional OLED 1 includes the following elements in top-down sequence: an encapsulating layer 12, which isolates the OLED 1 from the environment; a cathode 14; an organic light emitter 16; a transparent anode 18; and a transparent substrate 20, whose material may be a glass or a transparent plastic. In a general OLED, only the anode is transparent; however, there is also OLED having a transparent cathode or having transparent anode and cathode. FIG. 1(a) is the schematic diagram of the detailed structure of the organic light emitter 16. As shown in FIG. 1(a), the organic light emitter 16 includes the following elements in top-down sequence: an electron injection layer (EIL) 161, an electron transport layer (ETL) 162, an electro-luminescent (EL) layer 163, a hole transport layer (HTL) 164, and a hole injection layer (HIL) 165. Except the EL layer 163, other layers of the organic light emitter 16 are optional, depending on designer's choice.
The structures of full color OLED display devices can be divided into a stack one and a parallel one. Referring to FIG. 2(a) the schematic diagram of a stack structured full color OLED display device 3, herein, three OLEDs 32, 34, and 36, which emit red (R), green (G), and blue (B) lights respectively, are stacked on a substrate 38 to form a unitary full color pixel. There are three types of parallel structured full color OLED display devices. Referring to FIG. 2(b) the schematic diagram of the first type parallel structured full color OLED display device 4, herein, three OLEDs 42, 44, 46, which emit R, G, and B lights respectively, are disposed on a substrate 48 to form a unitary full color pixel. Referring to FIG. 2(c) the schematic diagram of the second type parallel structured full color OLED display device 5, herein, a white-light light source 52 in cooperation with three color filters 54, 56, and 58 creates R, G, and B lights. Referring to FIG. 2(d) the schematic diagram of the third type parallel structured full color OLED display device 6, herein, three color conversion elements 64, 66, and 68 convert the light emitted by a light source 62 and of a specific frequency into R, G, and B lights.
A few methods exist for fabricating OLEDs. Thermal evaporation is the de facto choice for fabrication of small molecular OLEDs. For fabrication of polymeric OLEDs, two approaches are commonly used. For monochrome OLEDs, the simple spin coating method is universally adopted. When addressing full color OLEDs, the inkjet printing method is the first choice coming to designer's mind. The conventional spin coating approach is a simple and inexpensive fabrication method; however, it cannot be utilized to fabricate full color OLEDs, as it can coat only one thin film on the substrate and lacks the ability to coat polymers into arbitrarily geometrical patterns. Feasibility of the inkjet printing method for fabrication of full color polymeric OLEDs was first demonstrated in [CBY98], which reported dual-color polymeric OLED pixels involving a spin-coated EL layer with blue emission topped by inkjet printed EL dots with red-orange emission. Custom-design and careful selection of the EL materials are necessary for the success of the inkjet printed full color OLEDs.
Limitation of the thermal evaporation method to small-size OLED displays, inability of the spin coating approach for full color OLED displays, and the fact that the inkjet printing technique is still at laboratory prototyping stage prompt many activities on alternative methods. Proposals directly addressing patterning of the EL layer for fabrication of full color or multi-color OLED displays include methods of thermal transfer [WBF03, HS02, CSS01 and references therein], electrochemical polymerization [ZWW03], photolithography using UV curable EL polymers [MFR03], screen printing [BBH01], and photolithography based on a new photoresist of a photoacid generating material and heat labile monomers [She01]. In the following, a brief review of these alternative methods is prepared by calling upon each method to selectively deposit the R, G, and B EL layers as shown in FIG. 3(a) which depicts a half complete, parallel structured full color OLED consisting of a substrate 102, an anode layer 104, an optional HIL 122, an optional HTL 124, and three discretely deposited EL patterns 126, emitting R, B, and G lights respectively.
FIG. 3(b) illustrates how discrete deposition of an EL pattern is achieved using the thermal transfer method. The key component of the thermal transfer method is a donor element 400 which, in one of many possible embodiments [WBF03], consists of a donor substrate 401, a light-to-heat conversion layer 402, and a transfer layer 403. For our application, the transfer layer is made of an EL material. With light radiation 406 through a mask 405, a part 404 of the EL transfer layer is transferred onto the HTL 124 due to the heat converted by the light-to-heat conversion layer. The half complete full color PLED is accomplished by repeating the same process for another two EL patterns.
FIG. 3(c) describes how the electrochemical polymerization method operates. The substrate 102 with patterned anode 104 is used as the positive electrode. Mononers of the desired EL polymer are dissolved in the electrolyte 412. When a voltage source 416 is applied to the patterned anode and a negative electrode 414, the monomers are oxidized, resulting in positively charged polymers selectively deposited on the patterned anode. Neutralization of the positively charged polymers is not necessary but it does give rise to an OLED device with “superior” performance [ZWW03]. Since electrochemical polymerization requires deposition on the electrode, the fabricated OLED device can not contain either HIL or HTL layer. Repeating the same process for another two EL patterns makes the half complete full color OLED.
FIG. 3(d) shows how, with specially synthesized UV curable EL polymers, traditional photolithography is applied to fabrication of full color OLED devices. The UV curable EL polymers are soluble before UV curing and become insoluble when photochemically crosslinked. For OLED applications, the UV curable EL material of one type is spun coated on top of the HTL layer. UV radiation 426 is then applied through a mask 424. A discrete EL pattern 126 is created after washing away the uncured non-crosslinked part 422. Repeated applications of the photolithography process gives rise to the needed R, G, and B patterns.
FIG. 3(e) shows a schematic of the screen printing approach [BBH01]. A screen 434 made of polyester fabric is placed above the HTL layer at a pre-determined gap, called a snap-off distance 432. A photoresist layer is coated onto the screen and photolithographically patterned as shown 436. Deposition of an EL pattern is screen printed by applying a soft rubber squeegee 438 over a solution of EL material 439. Repeating the screen printing process with properly patterned photoresist layer render the discretely printed R, G, and B patterns.
FIG. 3(f) to FIG. 3(h) highlight development of a full color OLED device through successive applications of photolithographic process whose success is hinged upon the invention of a new photoresist which includes a photoacid generating material and heat labile monomers [She01]. The photoacid generating material releases acid when exposed to light. After light exposure (448, FIG. 3(f)), the photoresist 442 is heated to a predetermined temperature and the monomers are joined by acid labile links to form a polymer. A special feature of this polymer is its solubility in a solvent not containing water and active hydrogen. FIG. 3(g) shows a photolithographically patterned photoresist 452 over layers of cathode 444 and an EL material 446. Reactive ion etching is then applied to remove the unprotected portion of the cathode and EL layer, creating a needed EL pattern. The remaining photoresist 452 is finally removed. Note that the need of an etching process like the reactive ion etching prevents the inclusion of the HIL and HTL layers in the OLED devices. After creation of one EL pattern, layers of the EL material of second type 466, cathode 464, and photoresist 462 are deposited as shown in FIG. 3(h). The same photolithography plus etching process is repeated to create a second EL pattern.
From above review, the thermal transfer method seems most feasible, competitive, and mature. The methods of electrochemical polymerization and photolithography using UV curable electroluminescent polymers require specially synthesized EL polymers, possibly resulting in compromised electroluminescence efficiency. One additional drawback of the electrochemical polymerization method is its exclusion of the use of HIL and HTL layers in device design and optimization. The same limitation preventing usage of HIL and HTL layers exists in the method of photolithography based on a new photoresist of a photoacid generating material and heat labile monomers. In its early development stage, the screen printing method still has rooms for improvement in resolution and in the on/off current ratio of the OLED devices such made.
Accordingly, by utilizing a directional spin coating technology, the present invention proposes a method for fabrication of OLED array which overcomes the aforementioned inability of the conventional spin coating for fabricating full color OLEDs, wherein the superiorities of simplicity and low-cost of the spin coating method are still maintained.